Contamination of 8.2 ka cold climate records by the Storegga tsunami in the Nordic Seas

The 8200-year BP cooling event is reconstructed in part from sediments in the Norwegian and North Seas. Here we show that these sediments have been reworked by the Storegga tsunami – dated to the coldest decades of the 8.2 ka event. We simulate the maximum tsunami flow velocity to be 2–5 m/s on the shelf offshore western Norway and in the shallower North Sea, and up to about 1 m/s down to a water depth of 1000 m. We re-investigate sediment core MD95-2011 and found the cold-water foraminifera in the 8.2 ka layer to be re-deposited and 11,000 years of age. Oxygen isotopes of the recycled foraminifera might have led to an interpretation of a too large and dramatic climate cooling. Our simulations imply that large parts of the sea floor in the Norwegian and North Seas probably were reworked by currents during the Storegga tsunami.

† Klitgaard-Kristensen, et al. 4 ; Klitgaard-Kristensen, et al.We have tested the original calibration of the radiocarbon ages of the green moss fragments 11 , picked out of the tsunami deposits and believed to have been killed the day the tsunami happened 12 , with IntCal20 13 and the sequence function in OxCal v.4.4 14 .We found only a minor change compared with the original calibration in Bondevik, et al. 11 . i.
The unmodelled date of the mean of the green moss ages of 7300 ±20 14 C years BP is 8030-8180 cal BP (95.4 % level) -the same as in Bondevik, et al. 11 . ii.
The modelled date of the mean of the green moss ages of 7300 ±20 14 C years BP with the sequence function, including radiocarbon ages above and below the Storegga tsunami deposits, is 8080-8180 cal BP (95.4 % level) -this changed by 10 years from 8070-8180 cal yr BP in Bondevik, et al. 11 . iii.
The mean of the modelled date of the green moss age is 8139 cal yr BP with 1 σ = 27 yr.This could be written as 8140 ± 55 cal yr BP (2 σ range).

Calibration of radiocarbon ages and reservoir age correction
We used a ΔR value of −145 ± 35 years and the Marine20 calibration curve 15 to correct the radiocarbon ages to calendar years.The ΔR value for the open Norwegian Sea and North Sea was found from the radiocarbon ages of a group of 19 whales, as recommended by Bondevik and Gulliksen in Mangerud, et al. 16 .This ΔR value of −145 ± 35 years corresponds to 7 ± 11 years relative to the Marine04 calibration curve).We have used http://calib.org/marine/(last accessed 2023/12/06) to calculate the ΔR value relative to the Marine20 calibration curve.

Age-depth models for sediment cores using BACON
To produce the age-depth models for the sediment cores we used the program BACON, version 2.3.9.1 17 installed as a package in the statistical software called R 18 .For each of the cores we performed two models: Model 1 includes all or most of the dates (specified in each case below) with no prior assumptions.In Model 2 we used the options hiatus.depths= the lower boundary of the 8.2 ka layer and slump= the thickness of the 8.2 ka layer, and discarded dates found within the layer that could be redeposited.We placed the hiatus depth right below the layer, allowing for a gap or break in the sedimentation, and the slump at the depths of the layer allowing for an instantaneous sedimentation between the given depths.Documentation for the Bacon program is found at (https://chrono.qub.ac.uk/blaauw/manualBacon_2.3.pdf, last accessed 2023/12/06) 2.2 Age model for sediment core LINK 14, east of the Faroe Islands According to Rasmussen and Thomsen 6 , a sand layer is present between 114 and 116 cm depthinferred to be deposited by the Storegga tsunami.We used the ages as published 6 and added the three new dates of foraminifera, at depths of 108, 114 and 117 cm (Supplementary Table 3).
Supplementary Table 3|  The AAR-lab no. were published in Rasmussen and Thomsen 6 .The UB-lab no.dated at the 14 Chrono Centre at the Queen's University Belfast.* GRIP age of the Saksunarvatn Ash 20,21 .
Model 1 includes all the dates in Supplementary Table 3 and has no prior assumptions.We notice a slight «bump» -upward convex age-depth curve across the depth of the sand layer in question that could indicate re-sedimentation and older ages (Supplementary Fig. 6).The age of the upper boundary of the sand layer at 114 cm depth is in this model 8566 (8397-8749) cal yr BP, about 400 years older than the age of the Storegga green moss.
Supplementary Table 1| Overview of the sediment cores * Risebrobakken, et al.

5 #
10smussen and Thomsen 6 § Depths read out of the grid used in the simulations -accounts for changes in bathymetry since ca.8150 cal yr BP; Hill, et al.7Estimated, velocity based on simulations by Williams and Fuhrman 8 of boundary layer.†Grainsizelimits read off the Sundborg diagram 9 for flows one meter above sea bed.Grain size calculated from equations in Miller, et al.10for flows one meter above sea bed.
* # Consolidated sediments (silt and clay).§ Supplementary Methods 1. Age calibration and age-depth models 1.1 Re-calibration of the age of the Storegga tsunami event